80 Spanning Tree Concepts ELS100-24TX
Communicating Between Bridges
Periodically, all d evices running the S panning Tree Protoc ol on a network
transmit packets to each other “in care of” the Bridge Group Address
which all bridges share. When a bridge receives a frame sent to the
Bridge Group Address, the bridge’s Spanning Tree Protocol processes
the packet. Application software and other LAN segments ignore the
packet. Bridges commu nicate between eac h o ther in order to dete r m i ne
the Root Bridge.
Selecting a Root Bridge and Design ated Bridges
During communication between bridges, one bridge is determined to
have the lowest bridge identifier. This bridge becomes the Root Bridge.
After the Root Bridge has been selected , each LAN s egment loo ks for the
bridge that has the lowest cost relative to th e Root Br idge . These bridges
become Designated Bridges.
Selecting Designated Ports
Each Designated Bridge selects a Designated Port. This port is
responsible for forwarding packets to the Root Bridge.
Handling Duplicate Paths
When the active topology of the network is determined, all packets
between any two nodes in the network use only one path. Where a
duplicate path exists, the non-desi g nated port is put into a blocking state.
Remapping Network Topology
If there is a change in the network topology due to a failur e or the remo val
or addition of any acti ve components, th e a ctive topology als o changes.
This may trigger a change in the state of some blocked ports.
There are five (5) states that the ports can be in for spanning tree:
•Blocking: A port in this state does not participate in the transmission
of frames, thus preventing dupl ica ti on ar i sin g th r oug h m ul ti ple paths
existing in the active topology of the bridged LAN.
•Listening: A port in this state is preparing to participate in the
transmission of frames. The transmission of frames is temporarily
disabled in order to prevent temporary loops, which may occur in a
bridged LAN during the life ti me o f this state as the active t opo l ogy of
the bridged LAN changes.
•Learning: A port in this state is preparing to participate in the
transmission of frames.
•Forwarding: A port in this state is pa rti ci pa ti ng i n the transmission of
frames.
•Disabled: A port in this state do es n ot par ti ci pa te in the transmission
of frames or the operation of the spanning tree process.